In recent years, a pressure-sensitive adhesive composition that does not utilize a solvent and can be hot-melt has been recommended from the viewpoint of environmental issues, energy saving, resource saving, and the like. A styrene-isoprene-styrene block copolymer is widely used as an elastomer that is used for such a pressure-sensitive adhesive composition.
A pressure-sensitive adhesive composition is required to exhibit pressure-sensitive adhesive performance such as the initial adhesion, peel adhesion strength, and holding power. In particular, a pressure-sensitive adhesive composition that exhibits excellent initial adhesion at a low temperature and excellent holding power at a high temperature, has been desired. Various studies have been conducted in order to improve the pressure-sensitive adhesive performance.
For example, Patent Literature 1 and 2 disclose a pressure-sensitive adhesive composition that includes a styrene-isoprene-styrene block copolymer and a styrene-isoprene diblock copolymer.
The pressure-sensitive adhesive compositions disclosed in Patent Literature 1 and 2 exhibit relatively excellent peel adhesion strength at room temperature, but exhibit insufficient initial adhesion at a low temperature or insufficient holding power at a high temperature.
A pressure-sensitive adhesive label is produced by bonding a pressure-sensitive adhesive sheet to a release sheet, and cutting the pressure-sensitive adhesive sheet using a die so as to have the desired shape. When a pressure-sensitive adhesive composition that exhibits an inferior die-cutting capability is used, the pressure-sensitive adhesive sheet may be cut insufficiently, and the pressure-sensitive adhesive labels may adhere to each other, whereby the commercial value may significantly decrease, or the pressure-sensitive adhesive composition may adhere to the die, and the subsequent cutting process may be adversely affected.
Various studies have been conducted in order to improve the die-cutting capability of a pressure-sensitive adhesive composition.
For example, Patent Literature 3 discloses a pressure-sensitive adhesive composition that includes a styrene-isoprene-styrene block copolymer and a styrene-butadiene-styrene block copolymer. Patent Literature 4 discloses a pressure-sensitive adhesive composition that includes a styrene-isoprene-styrene block copolymer, a styrene-isoprene-diblock copolymer, and a tackifying resin, and discloses that the composition has a single glass transition temperature.
However, the pressure-sensitive adhesive compositions disclosed in Patent Literature 3 and 4 exhibit an insufficiently improved die-cutting capability, or exhibit an improved die-cutting capability, but exhibit decreased pressure-sensitive adhesive performance (i.e., do not exhibit both excellent pressure-sensitive adhesive performance and an excellent die-cutting capability).
In order to solve the above problem, Patent Literature 5 discloses an elastomer composition that includes an aromatic vinyl-isoprene block copolymer that includes two or more polyaromatic vinyl blocks, an aromatic vinyl-isoprene diblock copolymer, and polyisoprene having a specific weight average molecular weight in a specific ratio.